Conventionally, in the preparation of polyolefin pellets, a strand cutting method or an underwater cutting method has been in general use.
The strand cutting method is a method in which a molten polyolefin resin is extruded through a nozzle with a strand shape, cooled, and then cut to a certain length (see Patent Document 1).
In this preparation method, the pellets are likely to change shapes, and it is difficult to maintain the shapes of the pellets to be constant.
In this regard, a granulation device as set forth in Patent Document 1 has been proposed.
In this granulation device, a weight of a pellet per unit time is detected by a weight detecting device.
In addition, a slit valve for extruding a polyolefin resin with a strand shape is provided with the device for modulating a degree of openness of a slit valve, and the weight of a pellet per unit time is fed back to the device for modulating a degree of openness of a slit valve.
In the device for modulating a degree of openness of a slit valve, a difference between a weight of a pellet per unit time as detected by the weight detecting device and a normal value is calculated, and based on the calculated result, the degree of openness of a slit valve is modulated.
By this, as described in Patent Document 1, the shape of the pellets can be kept constant. However, the strand cutting method basically has a problem that the production capability is extremely lowered, as compared with the underwater cutting method.
The underwater cutting method is a method in which a cutter in contact with the die surface rotates at a high speed, whereby a resin emerging from the die orifice is continuously cut at a high speed. Water usually flows in the cut segment, and accordingly, the cut resin and water are allowed to flow together, and finally to be dehydrated and dried. For example, Patent Document 2 describes that abrasion of a cutter knife can be prevented, and the generation of plugging can also be prevented by controlling the pressure of a cutter knife. Further, Patent Document 2 does not disclose a granulated pellet shape.    [Patent Document 1] JP-A-11-254431    [Patent Document 2] JP-A-5-228923